In the prior art, a cryogenics-based air separation unit generally comprises main heat exchangers with brazed plates which form the main heat exchange line of the cryogenics-based air separation unit.
These heat exchanges place in a heat-exchange relationship, on the one hand, air at room temperature and, on the other hand, cryogenic fluids coming from one or more distillation columns. At the output of such a heat exchanger, the air has a temperature of the order of −175° C., whereas the reheated fluids are roughly at room temperature (approximately 25° C.). Therefore the thermal gradient is approximately 200 K between the input and the output of a heat exchanger and the mean logarithmic temperature deviation is between 2 K and 10 K.
Each heat exchanger comprises a stack of parallel plates delimiting fluid passages, and spacers or heat exchange waves defining channels for these fluids. Peripheral closure bars ensure the seal-tightness of the fluid passages.
As is known per se, such a heat exchanger is overall in the form of a rectangular parallelepiped. The length of such a heat exchanger is typically from 4 to 8 m, its width from 1 to 1.5 m and its height from 1 to 2 m.
By convention, the length of a heat exchanger is the largest dimension of the parallel plates delimiting the fluid passages. The width of a heat exchanger is measured at right angles to the length. The height of a heat exchanger is measured in the direction of stacking of its plates.
Moreover, it is also known practice to increase the height of such a heat exchanger by assembling, for example by welding side-by-side, a number of separately-brazed exchangers, something which is not possible for increasing the length or the width.
The state of the art for such heat exchangers is to produce a counter-current heat exchange with a direction of flow of fluids in the lengthwise direction so as to benefit from the greatest dimension to produce the heat exchange.
FR-A-2844040 proposes using such an exchanger with a direction of flow of the fluids in the widthwise direction so as to considerably reduce (typically by a factor of 4 to 6) the number of exchangers to be arranged in parallel.
Nevertheless, to be able to achieve a thermal gradient of the order of 200 K with a low temperature difference and the most efficient exchange spacers-waves (for example so-called serrated waves having a short serration length and a very high density), it is necessary to increase the width of the exchanger to 2.5 m or even 3.5 m. Now, such a width of the exchanger is incompatible with all the existing brazing furnaces. Moreover, increasing the size of the brazing furnace would pose technical feasibility problems.
To remedy this problem, WO-A-2007149345 describes a heat exchanger assembly comprising two juxtaposed heat exchangers. In this case, the number of exchangers to be brazed is reduced only by a factor of 2 to 3, a factor which is all the same very significant.
Furthermore, the heat exchanger assembly of WO-A-2007149345 comprises means for fluidically connecting the juxtaposed heat exchangers. In the case in point, the primary fluid is high-pressure compressed air and the secondary fluid is low-pressure dinitrogen.
However, between the heat exchangers of WO-A-2007149345, the primary fluid is collected by oblique so-called distribution spacers which direct the secondary fluid to two lateral supply boxes (one on each side of the heat exchanger) and which have a small discharge section, which generates significant head losses. Similarly, the primary fluid is supplied to the second heat exchanger by two lateral supply boxes and oblique distribution spacers, which generates significant head losses.
Therefore, to neutralize this increase in the head losses, it would be necessary to increase the exchange sections. However, the dimensions of a heat exchanger are limited by the dimensions of the brazing furnace, in which this heat exchanger is manufactured. Therefore, such a heat exchanger assembly would entail brazing more exchangers and increasing the quantity of material needed to produce them.